This invention relates to processes for the production of Zintl compounds, processes for the production of intermetallic compounds and processes for manufacturing electronic components including intermetallic compounds. In particular, this invention relates to the application of intermetallic compounds in the manufacture of electronic components. More specifically, this invention relates to a method of applying intermetallic compositions onto the surface of electronic components.
Intermetallic compounds and Zintl phases have been known for some time. Zintl compounds are binary compounds formed between alkali or alkaline earth elements and post transition elements [(see for example, xe2x80x9cChemistry, Structure, and Bonding of Zintl Phases and Ionsxe2x80x9d, Ed. Susan M. Kauzlarich, VCH Publishers, (1996)]. One of the earliest examples of Zintl ions were those formed by the reaction of sodium in liquid ammonia with a variety of Group 14 metals, such as lead, to form, e.g. 4[Na(NH3)n+][Pb9]4xe2x88x92. These complexes are unstable due to the facile liberation of NH3, which can occur at low temperatures to form intermetallic compositions of the type NaPbx.
The isolation of solid derivatives of Zintl anions using ethylenediamine in the place of ammonia has been reported in which the alloy composition NaSn2.4.2.5, on slow dissolution in warm ethylenediamine followed by precipitation on the addition of THF or monoglyme, generates the species Na4(en)7Sn9 (en=ethylenediamine).
Macrocyclic ligands, such as 2,2,2-crypt, have been used in place of ammonia or ethylenediamine due to their effective sequestering capabilities. The stability of these cryptate complexes, an example of which includes [2(2,2,2-crypt-K)+[Pb5]2+, have enabled extensive characterisation of their crystal structures.
However, hitherto, the available techniques for producing Zintl compounds, have been cumbersome, especially where it is desired to produce Zintl compounds with predetermined stoichiometries. For example, the majority of these compounds have been obtained by dissolving pre-formed stoichiometric alloys of metals in ammonia. This route, which is required to obtain stoichiometric control of the product, involves high-temperature methods and highly specialised techniques. As a result, Zintl compounds, particularly of the heaviest (most metallic) post-transition elements, have generally only been prepared in very small scale (10-50 mg) and have therefore not been broadly accessible to the majority of synthetic chemists or useful in industrial processes.
As indicated, the invention also relates to processes for producing intermetallic compounds. Intermetallic compounds, which may be defined as mixed metal compounds of the type M1xM2y . . . M3z, possess properties that do not necessarily resemble the respective alloys and often exhibit properties which are intermediate between their component elements.
The majority of intermetallic alloys behave as semi-conductors and have thus found extensive applications in the electronics industry. Some intermetallic compounds display photoactive properties and these have been employed in photodetector components. The properties of the intermetallic layer are dependent upon the stoichiometry of the metal components. Thus the stoichiometric control of the metal components is important in order to achieve the desired electrical properties of the intermetallic layers.
The existing process for the manufacture of electronic components such as vacuum photodiodes having intermetallic layers based on antimony and alkali metals, involve the high temperature formation of antimony/alkali metal intermetallic layers using metal vapours. This deposition process typically involves predepositing an antimony layer onto the surface of the electronic component, followed by the addition of an alkali metal in vapour form. This process is highly labour intensive and the characteristics of the intermetallic films deposited are often variable as a consequence of inherently poor control of their stoichiometry.
It would therefore be highly desirable to improve the method in which intermetallic layers can be deposited onto electronic components during their manufacture. In addition, it would be advantageous to obtain a higher degree of stoichiometric control than those offered by the existing vapour deposition method.
Thus, it is a further object of the present invention to provide an improved process of forming intermetallic layers for use in the manufacture of electronic components. Such electronic components include photomultipliers and other photodetectors used in, e.g., medical scanners and scientific instruments.
Another object of the present,invention is to provide a method of producing intermetallic layers in which the stoichiometry can be controlled to furnish films with essentially consistent characteristics.
The present invention in all its aspects followed from the development of a novel procedure for producing Zintl compounds from stable precursors. This procedure enabled for the first time the production of Zintl compounds by a convenient route in a manner which permitted the Zintl compounds to be produced with preselected stoichiometries between the metal components thereof.
Advantageously, the use of a stable precursor to generate a Zintl compound that may be subsequently converted to an intermetallic alloy according to the present invention, allows the possibility for the deposition of the intermetallic alloy from solution at low temperature. Such a method provides a substantial improvement over existing vapour phase deposition techniques. The method of the present invention provides a convenient route to the formation of Zintl compounds on a gram or multi-gram scale.
Thus, the invention provides a novel application of Zintl compounds, especially when produced in accordance with the first aspect of the invention, in the manufacture of electronic components having surface coatings formed from intermetallic compounds.
According to the present invention, there is provided a process for the production of a Zintl compound comprising subjecting a heterometallic phosphinidene complex to thermal decomposition.
The heterometallic phosphinidene complex typically comprises at least two metals. Preferably one of the metals may be a metal of Group 13, 14 or 15 of the Periodic Table. Particularly preferred metals are those from Group 15 of the Periodic Table, including As, Sb and Bi.
The second of the metals is preferably a metal of Group 1 of the Periodic Table, e,g, Li, Na, K, Rb or Cs.
The heterometallic phosphinidene complex used in the process of the invention preferably contains one or more phosphinidene ligands [PR], which may be the same or different. The phosphorus atom of each phosphinidene ligand is covalently linked to a substituted or unsubstituted hydrocarbyl group, R.
The unsubstituted or substituted hydrocarbyl group, R, typically contains 1 to 15, preferably 4 to 10 carbon atoms, and may be selected from alkyl, alkenyl, alkynyl, cycloalkyl, aryl, aralkyl, and alkaryl. Typical substituents include F and alkylsilyl groups.
A particularly preferred phosphinidene ligand is PCy (Cy=cyclohexyl, C6H11), wherein the phosphorus atom is linked to a cyclohexyl group. Other examples of possible substituents include tBu [tertiary butyl, (CH3)3Cxe2x80x94], iPr [isopropyl, (CH3)2CHxe2x80x94], bis(trimethylsilyl)methyl [(CH3Si)2CHxe2x80x94], tris(trimethylsilyl)methyl {[(CH3)3Si]3Cxe2x80x94}, trimethylsilyl [(CH3)3Si] and fluorinated groups such as pentafluorophenyl (C6F5).
The phosphorus atom of each phosphinidene ligand in the heterometallic phosphinidene complex is generally coordinated to four metal atoms. It is preferred that the phosphorus atom is coordinated to three Group I metal atoms and one metal atom of Groups 13, 14 or 15.
The heterometallic phosphinidene complex may further comprise an additional ligand or ligands which may be Lewis bases. The structure of the complex will depend upon the identities of the metals and ligands.
Thus the preferred phosphinidene complexes referred to above can have various structures depending on the Group 13, 14 or 15 metal(s) present and on the alkali metal(s) and Lewis base ligand(s). Generally, however, the phosphinidene ligands (PR) are bridged between the Group 13, 14 or 15 metals and the alkali metals (i.e. the complexes will contain M1xe2x80x94P(R)xe2x80x94M2 groupings; M1=Group 13-15 metal, M2=Group 1 metal) and the phosphorus atom of the phosphinidene group will generally be coordinated by up to four metal atoms (for example, one Group 13, 14 or 15 metal and three alkali metals, as in Compound I).
The Group 13, 14 and 15 metals will usually be in their +3, +2 and +3 oxidation states respectively.
As indicated, the heterometallic phosphinidene complex may further comprise an additional, Lewis base ligand coordinated to one or more of the metal atoms. Preferred Lewis base ligands are primary, secondary or tertiary amines of formula R1R2R3N, wherein each of R1, R2 and R3 represent hydrogen, a C1-C10 alkyl or C6-C10 aryl group. In addition other Lewis bases such as polyamines [e.g. ethylenediamine (H2NCH2)2], permethylated polyamines (for example TMEDA {[(CH3)2NCH2]2} and PMDETA {[(CH3)2NCH2CH2]2NCH3}), pyridine (C5H5N) or polypyridines [such as 2,2xe2x80x2-bipyridine (C5H4N)2], may be employed. Oxygen donors such as ethers (e.g. tetrahydrofuran, THF) may also be used.
The thermal decomposition process of the invention may result in the co-production of a phosphorus compound having at least one Pxe2x80x94P bond, e.g. a cyclic phosphinidene. The thermal decomposition typically involves a so-called xe2x80x9creductive phosphinidene couplingxe2x80x9d or xe2x80x9creductive eliminationxe2x80x9d in the conversion of the phosphinidene intermediate into the Zintl complex. In such instances, the formation of phosphorus-phosphorus bonds, which have the highest bond energy between any Group 15 elements, provides the necessary thermodynamic driving force for the reaction to take place. As indicated below, a cyclic phosphinidene compound may be formed as a by-product and as will be appreciated, such a by-product will contain more than one Pxe2x80x94P bond.
Thus in one embodiment, the present invention comprises subjecting a heterometallic phosphinidene complex to a thermal decomposition reaction, wherein the heterometallic phosphinidene comprises:
(i) a plurality of phosphinidene ligands, [PR] wherein each R which may be the same or different, is selected from substituted or unsubstituted C1-C15 alkyl, alkenyl, alkynyl, cycloalkyl, aryl, aralkyl, and alkaryl, wherein the substituents may be F or alkylsilyl groups,
(ii) at least one metal selected from a Group 13, 14 or 15 metal;
(iii) at least one Group I metal selected from Li, Na, K, Rb, Cs; and
(iv) a plurality of labile, Lewis-base stabilising ligands, each of which may be the same or different, (and preferably as described above)
and said thermal decomposition process results in the co-production of a cyclic phosphinidene compound [PR]n, wherein n=4-6.
A particularly preferred process according to the invention comprises subjecting a heterometallic phosphinidene complex to a thermal decomposition reaction, wherein the heterometallic phosphinidene complex comprises:
(i) a plurality of phosphinidene ligands, [PR] wherein each R which may be the same or different,-is selected from substituted or unsubstituted C1-C15 alkyl, alkenyl, alkynyl, cycloalkyl, aryl, aralkyl, and alkaryl, wherein the substituents may be F or alkylsilyl groups,
(ii) at least one metal selected from Ge, Sn, Pb, As, Sb and Bi.
(iii) at least one metal selected from Li, Na, K, Rb and Cs,
(iv) a plurality of labile, Lewis-base stabilising ligands, each of which may be the same or different,
and said thermal decomposition process results in the co-production of a cyclic phosphinidene compound [PR], wherein n=4-6.
In the above embodiments R preferably contains 4 to 10 carbon atoms.
Preferably the phosphinidene compound has the formula (M1)n(M2)m(Lg)p(PR)r, wherein M1 is a metal of Group 13, 14 or 15 of the Periodic Table, M2 is a metal of Group 1 of the Periodic Table, PR is a phosphinidene ligand, Lg is a Lewis base ligand and each of n, m, p and r are in the range 1 to 10.
In the above formula, M1 is preferably As, Sb and Bi and M2 is preferably Li, Na, K or Rb. The group xe2x80x9cRxe2x80x9d in the phosphinidene ligand (PR) is preferably as defined above. Also, in the above formula, Lg is a Lewis base ligand which may be a primary, secondary or tertiary amine of formula R1R2R3N, wherein each of R1, R2 and R3 represent hydrogen, C1-C10 alkyl or C6-C10 aryl group. In addition other Lewis bases such as polyamines [e.g. ethylenediamine (H2NCH2)2], permethylated polyamines (for example TMEDA {[(CH3)2NCH2]2} and PMDETA {[(CH3)2NCH2CH2]2NCH3), pyridines (C5H5N) or polypyridines (such as 2,2xe2x80x2-bipyridine, (C5H4N)2 may be employed. Oxygen donors such as ethers (e.g. tetrahydrofuran, THF) may also be used.
In exemplary phosphinidene compounds, the ratios of m:n may vary from 4:1 to 1:4. Typically, m and n are 1, 2, 3 or 4, thus in phosphinidene compounds which are especially useful as starting materials, m may be 1 and n may be 3. In other phosphinidene compounds of interest, m is 1 and n is 2, or m and n are both 1. The integers p and r are typically in the range 4 to 8 and in exemplary compounds, both p and r are 6.
Especially preferred heterometallic phosphinidene compounds suitable for use in the process of the invention may be represented by the formula [Sb(PCy)3]2Li6.6Me2NH.2C6H5CH3 (I), {[cyclo-(CyP)4Sb]Na.Me2NH.TMEDA}2 (IV) and {[cyclo-(tBuP)3As]Li.TMEDA.THF} (VI).
It is preferred that the thermal decomposition process is carried out at a temperature of greater than 20xc2x0 C., and preferably at a temperature range of between 25-100xc2x0 C. The decomposition is preferably carried out with the phosphinidene compound in solution in an organic solvent such as a hydrocarbon (e.g. an n-alkane, such as n-hexane), an aromatic hydrocarbon (e.g. toluene) or tetrahydrofuran (THF).
Where the heterometallic phosphinidene complexes of the invention either contain stabilising Lewis base ligands, or stabilising Lewis base ligands which are aprotic (e.g. TMEDA, PMDETA, pyridines, polypyridines and oxygen donors such as ethers), the decomposition process may require activation. The activation procedure typically comprises subjecting the heterometallic phosphinidene complex to thermal decomposition as described above, in the presence of a primary or secondary amine (e.g. Me2NH) added separately to the complex.
In preferred embodiments of the invention, the thermal decomposition process results in the formation of a Zintl compound comprising a polymetallic anion consisting of atoms of one or more metals. In many instances, the polymetallic anion is coordinated with metal cations, and the metal cations are coordinated with Lewis base ligands. The Zintl compounds formed typically comprise a polymetallic anion consisting of atoms of a metal M1, wherein said polymetallic anion may or may not be coordinated with cations of a metal M2, and the cations of a metal M2 are coordinated with Lewis base ligands Lg. In other examples of Zintl compounds, the cation may be separated from the polymetallic anion. Thus as described in xe2x80x9cPolyatomic Zintl Anions of the Post-Transition Elementsxe2x80x9d, Corbett, J. D., Chem. Rev., 1985, 85, 385-397, a wide variety of different structures is possible. Thus where the ligand:alkali metal cation ratio is high, or when polyether or cryptand ligands are present, the cations may be xe2x80x9cseparatedxe2x80x9d by their coordination to the Lewis base ligands, so the cation is separated from the polymetallic anion.
Preferably the Zintl compound has the formula (M1)nxe2x80x2(M2)mxe2x80x2(Lg)pxe2x80x2, wherein M1, M2 and Lg are as defined above and each of nxe2x80x2, mxe2x80x2 and pxe2x80x2 are in the range 1 to 10. M1 is preferably a metal of Group 13, 14 or 15 of the Periodic Table. Especially preferred metals are those in Group 15, in particular As, Sb and Bi. M2 may be a metal of Group 1 of the Periodic Table, preferably Li, Na, K or Rb. Lg is preferably a Lewis base ligand which may be a primary, secondary or tertiary amine of formula R1R2R3N, wherein each of R1, R2 and R3 represent hydrogen, C1-C10 alkyl or C6-C10 aryl group. In addition other Lewis bases such as polyamines [e.g. ethylenediamine (H2NCH2)2], permethylated polyamines (for example TMEDA {[(CH3)2NCH2]2} and PMDETA {[(CH3)2NCH2CH2]2NCH3}, pyridine (C5H5N) or polypyridines (such as 2,2xe2x80x2-bipyridine, (C5H4N)2 may be employed. Oxygen donors such as ethers (e.g. tetrahydrofuran, THF) may also be used.
In exemplary Zintl compounds, the ratios of mxe2x80x2:nxe2x80x2 may vary from 5:1 to 1:5. Typically, mxe2x80x2 and nxe2x80x2 are 4-9, thus in the Zintl compound of formula (II) nxe2x80x2 is 7 and mxe2x80x2 is 3. The integer pxe2x80x2 depends on the metal M2 and the number of donor atoms in Lg and may be 1-8. The precise stoichiometry of the Zintl compound will depend on the nature and identity of the phosphinidene compound used as starting material (or where more than one phosphinidene compound is used, also on the ratio of different phosphinidene compounds used) and also on the identity of the Lewis acid base. Thus, by varying these parameters, a range of different Zintl compounds may be produced.
In accordance with a second aspect of the invention there is provided the use of a stable heterometallic phosphinidene complex as described above as a precursor for the deposition of a film containing an intermetallic compound.
In accordance with a third aspect the invention there is provided a process for the production of an intermetallic compound comprising:
(a) forming a Zintl compound by a method described above, the Zintl compound comprising a polymetallic anion consisting of atoms of a metal M1, and cations of a metal M2 which are coordinated with stabilising ligands Lg, and
(b) subsequently removing the stabilising ligands.
It is preferred that the stabilising ligand is a Lewis base, for example, one of the preferred Lewis bases referred to above. Advantageously, the stabilising ligands may be removed under reduced pressure or by evaporation at atmospheric pressure.
According to the invention, a preferred process for the production of an intermetallic alloy comprises:
(a) forming a Zintl compound by subjecting a heterometallic phosphinidene complex as described above to thermal decomposition, said Zintl compound comprising a polymetallic anion consisting of atoms of a metal M1, and cations of a metal M2 which are coordinated with stabilising ligands Lg, and
(b) subsequently removing the stabilising ligands.
A further embodiment of the invention provides a method for forming an intermetallic layer on a surface or surface portion of an electronic device, which method comprises:
(a) applying a Zintl compound to the surface, said Zintl compound comprising a polymetallic anion consisting of atoms of a metal M1, and cations of a metal M2 which are coordinated with stabilising ligands Lg, and
(b) subsequently removing the stabilising ligands.
Another embodiment of the invention provides a method for forming an intermetallic layer on a surface or surface portion of an electronic device, which method comprises:
(a) applying a heterometallic phosphinidene compound to the surface, said heterometallic phosphinidene compound being as defined above in connection with the general and preferred aspects of the invention,
(b) subjecting the heterometallic phosphinidene compound to thermal decomposition with concomitant loss of stabilising ligands.
It will be appreciated that in the above-mentioned thermal decomposition, there will normally initially be formed a Zintl compound, said Zintl compound comprising a polymetallic anion consisting of atoms of a metal M1 and cation of a metal M2 which are coordinated with stabilising ligands, and that the stabilising ligands will subsequently be lost.
The formation of the Zintl compound and the subsequent loss therefrom of the stabilising ligands may be a single or a two step process. Where the formation of the Zintl compound and removal of the stabilising ligand occurs in two steps, this embodiment of the invention may be defined in terms of a method for forming an intermetallic layer on a surface or surface portion of an electronic device, which method comprises:
(a) applying a heterometallic phosphinidene compound to the surface, said heterometallic phosphinidene compound being defined above in connection with the general and preferred aspects of the invention,
(b) subjecting the heterometallic phosphinidene compound to thermal decomposition to a Zintl compound, said Zintl compound comprising a polymetallic anion consisting of atoms of a metal M1 and cation of a metal M2 which are coordinated with stabilising ligands, and
(c) subsequently removing the stabilising ligands.
A still further embodiment of the invention provides a method of manufacturing an electronic device having an intermetallic layer on a surface portion thereof, which comprises:
(a) applying a Zintl compound to the surface, said Zintl compound comprising a polymetallic anion consisting of atoms of a metal M1 and cation of a metal M2 which are coordinated with stabilising ligands Lg, and
(b) subsequently removing the stabilising ligands.
In the above described methods for forming an intermetallic layer on a surface or a surface portion of an electronic device, the application of a Zintl compound or a heterometallic phosphinidene compound to such a surface, is preferably carried out by a technique such as spin coating, dip coating, vacuum evaporation or electrospray.
In these embodiments, the precise manner in which the polymetallic anion and the metal M2 are arranged will depend on the identities of M1 and M2 and the identities of the ligand(s). In many instances, the polymetallic anion will be coordinated with cations of metal M2.
As it will be appreciated, the ratio of metals in the heterometallic phosphinidene compound can be varied depending upon the ratio of starting materials and conditions in which they are synthesised. The thermal decomposition process of the invention will therefore produce a Zintl compound whose ratio of M1 and M2 (if not necessarily the same as that present in the phosphinidene compound) will be dictated by the individual chemical pathway of the decomposition.
Subsequent removal of the Lewis base stabilising ligands from the Zintl compound will produce an intermetallic compound having a metal composition which is therefore dictated by that of the heterometallic phosphinidene compound. Thus an important feature of the process of the invention is the possibility of selecting a heterometallic phosphinidene compound to produce an intermetallic compound having the desired metal: stoichiometry. As discussed above, the photoactive properties of intermetallic compounds are determined by the stoichiometries of the metal components.
The nature and composition of the intermetallic layer may be adapted to confer desired electrical properties depending upon the electronic device for which it is to be applied to. By using different phosphinidene precursors or Zintl compounds or mixtures thereof, intermetallic layers with different proportions of metals may be formed. Intermetallic layers, in particular, those comprising an alkali metal and one or more metals selected from those of Group 13, Group 14 or Group 15 of the Periodic Table (especially in stoichiometric quantities) are particularly useful in such applications, as they possess highly desirable photoactive characteristics. Particularly useful is the application of intermetallic layers having electron emitter properties, such as those required by photoelectric devices. For example, intermetallic alkali metal antimonide films have wide applications as batteries and photoactive materials in photomultipliers which are used e.g. (1) medical scanners, (2) scientific instruments, and (3) particle physics.
Structural formulae of exemplary compounds are given in the accompanying drawings (FIGS. 1-6).